1. Field of the Invention
This invention relates generally to an improved plating barrel design and more particularly to method and apparatus for providing an electrical contact to a plating barrel electrode.
2. Prior Art
One technique for electroplating small or medium size metal parts is to immerse those parts in a plating bath rich in charged ions and cause electrodeposition of the ions onto the parts. During such a process the parts act as a cathode and supply electrons which combine with the charged ions.
The use of plating barrels for electrodeposition is known in the electroplating art. The parts are placed into a plating barrel which is immersed into a plating vat containing the ion-rich solution. As the plating barrel is rotated in the bath, the plating solution flows through holes in the barrel walls in contact with the parts. The metallic pieces or parts inside the barrel are maintained at a constant potential and current flow to those parts is produced by applying a higher potential to the plating solution. So long as the metallic parts are maintained at a constant potential and adequate mixing of the solution is achieved, electrodeposition will occur.
Various mechanisms have been used to maintain the metallic parts in the plating barrel at a constant potential. One technique discussed in U.S. Pat. No. 3,256,170 to Neilson is to provide a buss bar system whereby a plurality of buss bars are positioned about an interior barrel surface and contact the metallic parts during barrel rotation.
Two other techniques for maintaining the parts at a constant potential are disclosed in U.S. Pat. No. 3,663,410 to Schumacher. One technique is to insert a dangler type cathode conductor into the barrel and allow the parts to come in contact with the dangler. A second alternative technique disclosed in the Schumacher patent is to mount a conductive disc type cathode to a barrel end wall so that regardless of the barrel orientation the parts contact a portion of the disc.
The above three cathode arrangements work with varying degrees of success, but suffer from a common deficiency. The electrical coupling between the source of constant potential outside the barrel and the cathode is subject to failure.
The preferred prior art technique for such coupling is a trunnion type arrangement where a conductive bar is mounted inside a rotating sleeve which in turn is coupled to the cathode. During use coaction between the rotating sleeve and the stationary bar causes wear on one or both of the relatively rotating members. As the wear continues, it is possible that contact between the sleeve and bar will be lost in which case electroplating will cease.
A second problem accompanying prior art contact design is that the contact between rotating members is not protected from corrosive attack by the electrolyte in and around the barrel. The electrolyte can wear away the electrical contact and cause either an open circuit condition or at least reduced plating effectiveness. The presence of electrolyte in the contact area can also cause metal electrodeposition on the contact which increases the electrical resistance of the contact area.